Bring a pot of salted water to a rolling boil. Fill a large bowl with ice water and set it next to the stove.
Add the green beans and cook for 4 to 5 minutes, until tender with a crisp snap.
Immediately scoop the beans into the ice bath to stop the cooking and lock in the bright green color. Drain well and pat dry.
Make the Dressing
In a small bowl, whisk together the olive oil, red wine vinegar, lemon juice, garlic, oregano, and Dijon mustard. Season with salt and pepper and set aside to let the flavors meld.
Assemble the Salad
Add the drained beans to a large bowl and toss with most of the dressing while they are still slightly warm so they soak up the flavor.
Gently fold in the cherry tomatoes, red onion, Kalamata olives, and red bell pepper. Toss just until coated.
Crumble the feta over the top and scatter the fresh parsley. Serve right away or let it rest, as it tastes even better after a sit.
Notes
Tame raw onion: Soak the sliced red onion in cold water for 10 minutes, then drain to soften the sharp bite.
Make ahead: The beans, onion, and olives can marinate in the dressing for up to 24 hours. Add the tomatoes, feta, and parsley closer to serving.
Storage: Keeps in the fridge for 1 to 2 days in a sealed container. Best eaten within a day, as the tomatoes soften over time.
Frozen beans: Cook a touch longer, then drain and pat very dry so they do not water down the salad.